The present invention relates to a bidirectional multimedia communication terminal.
Transmitting multimedia data, including at the same time sound or voice data, digital data and images, implies using a very wide frequency band and therefore media capable of transmitting such bandwidths. The media used today essentially comprise optical fibers, radio beams and satellites.
The use of optical fibers has the drawback that it is necessary to install a dense terrestrial infrastructure. The use of radio beams suffers from the same disadvantage, plus that relating to the sensitivity of this mode of transmission to interference due to the radio and atmospheric environments, which can degrade the quality of links, and even cut them off.
Geostationary satellites are routinely used at present to transmit multimedia data in the C or Ku band, in conjunction with VSAT terminals using antennas with a wide radio aperture and therefore a small geometrical diameter. There is of course no need to provide the antenna with a satellite tracking system, as the satellite is geostationary, but this advantage is greatly outweighed by the following disadvantages: 1) the satellite must have very high power transmitters to compensate the attenuation of the signals in space (the satellite is approximately 36 000 km from the Earth), and 2) there is a time-delay of more than 2 seconds for data making a return trip between the Earth and the satellite.
The use of non-geostationary satellites in low Earth orbit (at altitudes from 350 to 20 000 km, for example) eliminates or reduces the latter drawback by reducing the time-delay, for example to 100 ms. Nevertheless, in a multimedia application, data must be provided on the ground for tracking the non-geostationary, i.e. moving, satellite to maintain a call, which is interrupted when the satellite pointed at leaves the space in which it is xe2x80x9cvisiblexe2x80x9d from the ground.
One solution to this problem is to use a constellation of non-geostationary satellites in low-Earth orbit, instead of a single satellite. With a constellation including several hundred satellites (as in the TELEDESIC project), it is then possible to ensure continuous coverage of a given site by the Individual coverage areas of the satellites of the constellation, which avoids any recourse to means for tracking non-geostationary satellites. However, this kind of solution would not seem to be economically viable, given the cost of manufacturing, launching and maintaining a constellation comprising several hundred satellites.
One specific object of the invention is to provide a multimedia communication terminal adapted to constitute a communication interface between a satellite-based multimedia communication system and devices for receiving and/or sending multimedia data that does not have the drawbacks of the prior art means for achieving this and which in particular enables continuous bidirectional communication at high bit rates with a time-delay imperceptible to users, in the context of an economically viable communication system.
Another object of the invention is to provide a terminal of this kind that is designed to be autonomous, mobile and compact, so that it can be transported and installed easily on any user site.
The above objects of the invention, together with others that will become apparent on reading the following description, are achieved by a bidirectional multimedia communication terminal including at least two orientable antennas for sending/receiving radio signals conveying multimedia data, electronic signal processor means for processing signals received or to be sent via the antennas, pointing means associated with each antenna for pointing it at a predetermined satellite from a constellation of non-geostationary satellites forming part of a multimedia telecommunication system, and switching means for selectively switching the transmission of multimedia data between the terminal and the satellites of the constellation from the satellite at which one of the antennas is pointed to the satellite at which the other antenna is pointed, so as to guarantee continuous transmission of a data stream between the terminal and the constellation, the pointing means including an antenna orientation unit associated with each antenna and control means for controlling the orientation units on the basis of data on the position of the terminal in a frame of reference tied to the Earth and on ephemerides of visible satellites of the constellation supplied by a mission center of the constellation, which terminal further includes a system for delivering the data on the position of the terminal. The positioning system is advantageously connected to the GPS constellation to supply terminal position and time data to the control means. Similarly, the frame of reference tied to the Earth is advantageously defined by a geostationary satellite.
As described in more detail later, the resulting terminal provides a satellite-based multimedia data communication system having the advantageous features stated above.
Other features and advantages of the present invention will become apparent on reading the following description and examining the accompanying drawings.